We propose to graft strongly incompatible cardiac allografts into lethally irradiated random-bred dogs. Cryopreserved autologous marrow cells and thymus-derived lymphocytes will be infused 0 or 2 days later. Irradiated blood products (platelets, leukocytes) will be infused during the first few weeks prior to hemopoietic reconstitution. Stem cell recovery will be stimulated by nandrolone decanoate, an anabolic steroid. Survival times of heterotopic cardiac allografts and survival of dogs with orthotopic cardiac grafts will be the measure of success of this protocol. The model in the rat which led to the experiments designed for dogs will be used to evaluate the mechanism of "transplantation tolerance" and to develop improvements in the protocol to be applied to the dog model. For example, the LEW host grafted with marrow and thymus cells will be immunized with LEW idiotypic receptors for the WF heart donor's alloantigens.